A printed circuit board (PCB) is a composite of organic and inorganic materials with external and internal wiring, allowing electronic components to be mechanically supported and electrically connected. A PCB typically contains multiple insulation layers sandwiched between metal layers. A typical PCB contains at least one metal layer that is patterned to transfer signals between the electronic components, one power plane for distributing voltage throughout the PCB, and one ground plane.
The method and manner in which components are attached to a PCB has changed as integrated circuits and circuit board technology has improved. Today, a significant number of components are surface mounted to the PCB while others have pins soldered into plated-through-holes. In surface-mount technology, the components are soldered directly to the surface of a PCB via solder ball connections, in contrast to the through-hole mode in which the component leads are inserted through a PCB. The driving force for the use of surface-mount technology is the reduced package size, improved utilization of board real estate and assembly simplicity.
As integrated circuit technology has improved, substantially greater functionality has been incorporated into the devices. As integrated circuits have expanded in functionality, the size of the devices has diminished resulting in higher clocking frequencies and increased power consumption. As a consequence, the integrated circuit devices of today generate more heat while possessing smaller surface areas to dissipate the heat. Therefore, it is important to have a high rate of heat transfer from the integrated circuit package to maintain the junction temperatures of the integrated circuit within safe operating limits. Excessive junction temperatures may affect the performance of the circuit and cause permanent degradation of the chip. For this reason, heat sinks are typically mounted to the top surface of the package housing to increase the thermal performance of the package.
FIG. 1 illustrates a prior art method of attaching a heat sink to an integrated circuit package. FIG. 1 shows a surface mounted package 102 that is electrically coupled to a printed circuit board 104 via a plurality of solder ball interconnections 103. Generally, package 102 includes a heat slug 108 that is thermally coupled to the integrated circuit die that is housed within the package. Heat is removed from package 102 by a heat sink 106 that is thermally coupled to heat slug 108. A highly conductive thermal grease 110 is normally placed between heat slug 108 and heat sink 106. A thin layer of thermal grease 110 ensures that the thermal interface between heat slug 108 and heat sink 106 is devoid of any air gaps, which would reduce the heat removal capability of the packaged device. As shown in FIG. 1, a heat sink clip 112 is used to secure heat sink 106 to the packaged device. Clip 112 generally includes two legs 113a and 113b that terminate at hooked sections 114a and 114b. (Clip 112 is generally referred to as a "spider clip.") Attachment of the heat sink 106 to the package 102 is achieved by placing the heat sink clip on top of heat sink 106 and compressing legs 113a and 113b inwardly to enable the insertion of hooks 114a and 11b into through holes 116a and 116b formed within printed circuit board 104. Hooks 114a and 114b bear against the bottom side of printed circuit board 104 to provide a force to heat sink 106, forcing it to seat against heat slug 108.
As noted above, as integrated circuit technology has improved, substantially greater functionality has been incorporated into the devices. As a result, the integrated circuit devices of today generate more heat and require the use of heat sinks that are both larger and heavier than those used in the past. In order to meet certain shock and vibration performance requirements, a larger force must be applied to the heat sink to hold it securely in place.
Although the use of a spider clip to attach a heat sink to package has proved sufficient in the past, a number of problems arise when extending its use to the attachment of the larger and heavier heat sinks of today. First, the amount of force that may be exerted by a prior art spider clip is very limited. If the force necessary to hold a heat sink in place is too high, the spider clip hooks can easily become dislodged from the printed circuit board.
As described above, the force created by spring clip 112 causes hooks 114a and 114b to bear against the bottom side of the printed circuit board. A moment force is produced within the board since the forces acting on the printed circuit board are located at points away from the center of the package. Since the printed circuit board is flexible, this moment force causes the board to warp when hooks 114a and 114b bear against the bottom side of the board. This introduces a number of problems. First, warping or bending of the printed circuit board can cause a crack to form in the board. These cracks may cut metal traces in the board to create electrical opens. Another problem arises when the integrated circuit package attached to the board is a surface mounted device. As shown in FIG. 1, a surface mounted device is electrically coupled to the printed circuit board by a multitude of solder ball interconnections 103. The solder bumps are very small (approximately 4 to 8 mils in diameter) and are easily damaged. Excessive warping of the printed circuit board can cause these solder bump interconnections to fatigue or crack, thus resulting in an inoperable device.
Bending of the printed circuit board can also affect the thermal performance of the integrated circuit package. As previously discussed, a thermal grease is generally used at the heat sink/package interface to enhance the transfer of heat at the interface. The gap between the heat sink and package must be held to within a tight tolerance. The thickness of this gap, which is commonly referred to as the "bond line thickness", is chosen to account for the differences in the thermal expansion between the heat sink and package during the thermal cycling of the device. If the printed circuit board is permitted to warp, it may be impossible to maintain the proper bond line thickness which is critical to the thermal performance of the electronic device.
Therefore, what is needed is an apparatus for attaching a heat sink to an electronic package that solves the aforementioned problems.